[evlatests] 3bit vs 8bit flux transfer

Thu Jan 28 11:06:15 EST 2021

I fear something else is amiss with ea06, as the vertical beam profiles 
at some bands (but not all) are highly distorted.

Regarding stability -- there is abundant evidence that the Tcals (and 
efficiencies) are indeed very, very stable.  The 'gain histograms' I 
made on Tuesday are unchanged (in a statistical sense --- I haven't 
attempted a one-by-one comparison) with those I made about eight years 
ago.

In my opinion, we would do well to 'tighten up' the histograms by 
suitable adjustment of the Tcal values (or efficiencies, if we think 
appropriate).  After doing this, accurat flux calibration could easily 
be done with a single eight-bit observation on any calibrator.  No need 
for a long journey to an 'absolute' standard (which is most cases means 
hoping it hasn't changed...)

I'll spend some time today comparing 'absolute' 3-bit vs 8-bit with the 
data I have.  I expect good results, as the observations were made at 
the same elevation of sources with about the same flux.  So this may not 
be too meaningful in the general case (varying elevations, and widely 
varying input power).

Rick

On 2021-01-26 23:19, Barry Clark via evlatests wrote:
> The tcals for receiver X019 in ea06 were measured on 2013-nov-22.  The
> receiver was installed in ea06 on 2017-feb-16. Possible something
> happened to it when it was in the lab in 2017 that made it discrepant,
> and anyway, seven years stability is a lot to ask, though some things
> are still quite good at that age.
> 
> On 1/26/2021 5:15 PM, rperley via evlatests wrote:
>> A short test run was take a few days ago, which compared flux transfer 
>> accuracy between 3 and 8 bit quantizers, and which assessed the 
>> accuracy of 'absolute' flux density calibration with the 8-bit 
>> samplers.
>> 
>> Observations were made of 3C286 and J2007+4029 (the calibrator source 
>> for Cygnus A) when the two sources were at the same elevation.  Bands 
>> utilized were X, Ku, K, Ka, and Q.  For each band, observations were 
>> made in both wideband (3-bit) and narrowband (8-bit) modes.  For the 
>> latter, the two IFs were chosen to lie near the opposite ends of the 
>> bands.
>> 
>> Test1:  Compare flux transfer between the sources.
>> 
>>    For the 8-bit mode the full switched power values were applied to 
>> the data.
>>    For the 3-bit mode, only the requantizer (RQ) gains were applied, 
>> as it is known that the 3-bit digital switched power ('PDif') is not 
>> linearly related to the analog power.
>> 
>>   Result:  The determined flux density for J2007, based on 3C286, were 
>> the same, within 1%, for both sampler modes.
>>    This surprised me a bit -- I had expected a few percent 
>> differences.  Evidently, the actual gain variations of the 
>> amplifiers/IFs between the two sources (located far apart on the sky) 
>> is very small.  So small that in fact, no gain correction at all is 
>> required!  (At least, not for this day).
>> 
>> Test 2:  How good is the gain calibration when based on knowledge of 
>> the system gain constants (Tcal and antenna efficiency).
>> 
>>    For this, the full switched power values were applied to the 8-bit 
>> data, and 'CALIB' was run on the visibilities from 3C286. If 
>> everything is perfect, the resulting gains will all be 1.0.
>> 
>>    They weren't.
>> 
>>    But for most antennas, they were quite close.
>> 
>>    I plotted histograms of the voltage gains for the central SPW for 
>> the two IFs in the 8-bit data, each polarization separately. Note that 
>> the conversion from correlation to calibrated visibilitites uses the 
>> Tcal and efficiency values which are externally determined (i.e., they 
>> are tabled quantities).  The observed gain values will be greater than 
>> 1.0 when the tabled Tcal is too low, or the tabled efficiency is too 
>> high.  In other words, if the corrected visibility is too low, then 
>> the actual Tcal is higher than that listed, and/or the actual 
>> efficiency is lower than that listed.
>> 
>>    a) X-band.  For both frequencies (8.3 and 11.3 GHz), the mean gain 
>> values are between .99 and 1.01.  The spread is quite small (about 
>> 0.12).  One antenna is seriously discrepant -- ea06, whose gain value 
>> is about 1.3.
>>    For the higher frequency, the gain voltage spread is much wider, 
>> with the histogram very skewed to the high side.  Antennas 6, 21, 28, 
>> and 1 all have abnormally high required gains.  Due to the skew, the 
>> median gains are 0.96 and 0.98 for RCP and LCP, respectively.
>> 
>>    b) Ku-band.  The means and medians are between .98 and 1.02. ea06 
>> is again discrepant.  The spread in voltage gains is 0.1 to 0.2, with 
>> the higher spread at the higher frequency.
>> 
>>    c) K-band.  The means and medians are again .98 to 1.02, for both 
>> frequencies.  The spread is very small at 19 GHz (0.1) slightly higher 
>> at 25 GHz.  ea06 is better behaved here, but still too high.
>> 
>>    d) Ka-band.  At 32 GHz, the medians are about 1.01, but the means 
>> are higher, as there are some antennas with high corrections 
>> required:  ea25, ea01, ea19, and ea21.  All are above 1.2.
>>    At 37 GHz, the situation worsens.  The medians are about .98, but 
>> the same antennas as listed at 32 GHz are quite discrepant.
>> 
>>    e) Q-band.  The trends noted above continue.  Means and medians are 
>> both above 1.0 -- and antennas 1, 6, 19, and 25 are truly bad.  (They 
>> also have very bad vertical beam profiles).  Excluding the obvious 
>> deviants, the mean corrections at 42 GHz will be very close to 1.0, 
>> but at 48 GHz, it is about 1.1 -- a 20% error in flux.
>> 
>>   By adjusting the Tcal or efficiency values, the spread in the gain 
>> voltages can be greatly reduced, the medians (and probably the means) 
>> brought to within a couple percent of 1.0.  The trick will be to 
>> determine these adjusted values as a function of frequency.  There are 
>> a lot of frequencies!
>> 
>> 
>> _______________________________________________
>> evlatests mailing list
>> evlatests at listmgr.nrao.edu
>> https://listmgr.nrao.edu/mailman/listinfo/evlatests
> 
> _______________________________________________
> evlatests mailing list
> evlatests at listmgr.nrao.edu
> https://listmgr.nrao.edu/mailman/listinfo/evlatests